1. Field of Invention
The present invention relates to a method of inhibiting radioactive substances from eluting into the cooling water in a nuclear plant, such as a boiling water reactor (BWR) or an advanced converter, and to an apparatus therefor. More particularly, the present invention relates to a method of inhibiting radioactive substances from eluting into the cooling water in a nuclear plant in which the iron concentration in the cooling water is reduced smaller than 1 ppb, wherein the surface dosage in the primary system is reduced, and to an apparatus therefor.
2. Description of the Prior Art
A conventional method of inhibiting radioactive substances from eluting into the cooling water in the boiling water reactor has been disclosed, for example in Japanese Patent Laid-Open No. 79194/1986, according to which, when the boiling water reactor is placed in the business operation, the Fe/Ni molar concentration ratio in the boiling water reactor is controlled at a level of 2 to 8; however, no satisfactory results are produced therein.
The above conventional method is practiced under the observation by model analysis that the minimum surface dosage in the primary system is obtained after about 3000 hours from the start of operation of the boiling reactor. Therefore, the control of the Fe/Ni molar concentration ratio is practiced inevitablely after about 3000 hours from the beginning of the operation of the boiling water reactor.
Besides in another prior art method of inhibiting radioactive substances from eluting into the cooling water in the boiling water reactor, the iron concentration in the cooling water is gradually increased with the radioactive cobalt (.sup.58 Co) ion concentration in the cooling water as an index, nickel ions being brought into the nuclear reactor from the feed water system becoming the radioactive cobalt (.sup.58 Co) ions.
In the above mentioned conventional method, however, no consideration has been given to the iron crud, for example .alpha.-Fe.sub.2 O.sub.3, on the fuel cladding surface, and to the covering ratio at which the outer surface of the fuel rod including the fuel pellets therein, or the outer surface of the fuel cladding tube, is covered with the iron crud that affects the reaction of the iron crud with nickel and cobalt. The corrosion substance formed on the cladding tube is iron, cobalt or nickel, the radioactive substances being .sup.58 Co, .sup.60 Co or .sup.54 Mn.
During the initial stage of the operation cycle in the business operation and in case the nickel ion concentration has dropped greatly, furthermore, the index, i.e., the Fe/Ni molar concentration ratio, becomes no more effective in decreasing the .sup.58 Co ion and .sup.60 Co ion concentrations in the cooling water. Further, when it is attempted to increase the Fe/Ni molar concentration ratio, the iron crud may be introduced in an excess amount, whereby .sup.54 Mn ions formed by the reaction of .sup.54 Fe(n, p) often causes the surface dosage in the primary system to increase.
When the Fe/Ni molar concentration ratio on the fuel cladding surface increases, the activity of .sup.54 Mn crud or .sup.60 Co crud increases qualitatively and, however, .sup.60 Co ion or .sup.58 Co ion decreases qualitatively.
An object of the present invention is to provide a method of inhibiting radioactive substances from eluting into the cooling water in a nuclear plant, wherein the iron concentration in the cooling water can be suppressed, and the amount of sedimenting crud-like (insoluble) radioactive substances is as small as possible.
Another object of the present invention is to provide a method of inhibiting radioactive substances from eluting into the cooling water in a nuclear plant, wherein the iron concentration in the cooling water can be controlled to decrease the .sup.58 Co ion and .sup.60 Co ion concentrations in the cooling water.
A further object of the present invention is to provide a method of inhibiting radioactive substances from eluting into the cooling water in a nuclear plant, wherein the surface dosage in the primary system can be decreased at the time of regular checking contributing to decreasing the surface dosage in the primary system to which a worker may be exposed. In the present invention, the "crud" is defined as a particle substance which does not pass through the myriapore filter having 0.45 .mu.m pore. The "ion" is defined as a substance which passes through the myriapor filter having 0.45 .mu.m pore.
The above-mentioned objects are achieved by a method of inhibiting radioactive substances from eluting into the cooling water in a nuclear plant in which after a new fuel cladding, which is an unused one and is loaded into the nuclear reactor for the first time and further does not have material substance adhering on the fuel cladding surface, is loaded into the nuclear reactor. With nuclear heating, an iron ion source is injected into the cooling water in order to form an iron oxide layer on the fuel cladding surface; and after the iron oxide layer is formed on the fuel cladding surface at a covering ratio of 100%, an injecting amount of the iron ion source is lowered so that an Fe/Ni molar concentration ratio in the cooling water is set in a range of from about 2 to 10.
The iron ion or the iron crud is injected during the pre-operation to form the iron oxide layer on the fuel cladding surface at the covering ratio 100%. The iron concentration in the cooling water is controlled by controlling a flow rate of the cooling water through a condensed water by-pass line. The iron concentration in the cooling water is controlled by controlling an iron electrolyzing current.
Monitoring the amounts of iron, nickel and cobalt in the cooling water of the boiling water reactor makes it possible to learn the amounts of corrosion products such as iron crud, nickel ion and cobalt ion that are brought into the nuclear reactor within a period of time in which measurement is taken, and serves as an input that is necessary for calculating the amount of material substance adhered on the fuel cladding surface.
Further, estimating the amount of material substances adhered on the fuel cladding surface by calculation, and controlling the amount thereof, are effective for efficiently reacting the iron crud that builds up on the fuel cladding surface with nickel and cobalt, for reducing the .sup.58 Co ion and .sup.60 Co ion concentrations in the cooling water, and for inhibiting the amount of undesired radioactive crud such as .sup.54 Mn from increasing. The amount of the iron crud adhered on the fuel cladding surface can be decreased by controlling the ion concentration of the iron crud in the cooling water.
In accordance with the present invention, in an apparatus for inhibiting radioactive substances from eluting into the cooling water in a nuclear plant, the nuclear plant includes a nuclear reactor, a steam turbine, a condenser, a condensed water purifying unit, a feed water heater, a recirculation system, a means for measuring iron concentration in the cooling water on a downstream side of the feed water heater, a means for measuring nickel concentration in the cooling water on the downstream side of the feed water heater, a means for injecting iron into the cooling water on the downstream side of the condensed water purifying unit, and a control unit for controlling the iron concentration injecting means so as to adjust an amount of iron injected based upon a signal sent from the iron concentration measuring means and the nickel concentration measuring means and for representing an amount of the iron concentration.
A means for controlling the iron concentration in the cooling water is provided in the nuclear plant; the iron concentration controlling means comprise a means for evaluating a total amount of iron concentration adhered on the fuel cladding surface based upon the iron concentration in the cooling water, a means for controlling the amount of iron injected into the cooling water based upon the total amount of the iron concentration on the fuel cladding surface, and a means for supplying iron that is to be injected through the iron concentration injecting amount controlling means.
According to the present invention, .sup.58 Co ion and .sup.60 Co ion concentrations in the cooling water can be decreased without much increasing the concentration of precipitating radioactive crud such as .sup.60 CoFe.sub.2 O.sub.4, .sup.58 CoFe.sub.2 O.sub.4 or .sup.54 MnFe.sub.2 O.sub.4, and the surface dosage in the primary system can be decreased at the time of regular checking contributing to decreasing the surface dosage in the primary system to which a worker may be exposed.